The present invention relates to pipeline distortion monitoring. For either above-ground, under-ground, or submarine pipelines there is a possibility that, over time, geometric distortions may occur due to local earth heave or subsidence. If the pipeline distortion at some point becomes too large, there is the possibility of pipeline rupture with possible undesirable human safety, economic, and environmental consequences. To avoid the possibility of such undesirable consequences, a regular survey of the pipeline dedicated to assessing its geometric stability is highly desirable.
Distortion monitoring of pipelines is essential to forecasting integrity changes which allow corrective measures to be taken before actual pipeline failure occurs. Pipeline distortion monitoring has the unique objective of assessing the geometric stability of a pipeline and represents a totally different function than pipeline inspection which has as its objective the monitoring of the inner surface of a pipeline for corrosion and weld integrity. Pipeline distortion monitoring has a unique additional element distinguishing it from pipeline inspection. This is the element of time over which the pipeline shifts from one time epoch to another. Pipeline distortion monitoring requires a comparison of the present pipeline geometry to that which existed at an earlier epoch, and a determination of both the angular and linear shifts that have occurred in the pipeline at all points along its length.
A pipeline PIG is a mechanism that travels through a pipeline detecting profile, ovality, displacement of oil as well as sensing other features of the pipeline. Various prior art pipeline PIGs have relied on special hardware elements to measure distortion characteristics. Two concepts for pipeline distortion monitoring have used a strapdown attitude reference system to measure the attitude of a PIG relative to a local vertical frame of reference as the PIG traverses the pipeline, together with a secondary means which allows the attitude of the PIG relative to the pipeline to be determined. One such concept, described by Levine (U.S. Pat. No. 4,524,526), utilizes a mechanical arm which makes physical contact with the pipeline, and whose angular position with respect to the PIG body provides a measurement of the PIG attitude relative to the pipeline. A second concept, described by Adams et. al. (U.S. Pat. No. 4,945,775), utilizes sonar measurements to provide the PIG attitude relative to the pipeline. By either means, the PIG attitude relative to the local vertical frame, together with the PIG attitude with respect to the pipeline provided by the secondary means, allows the pipeline attitude relative to a local-vertical geographic frame to be determined at all points along the course of the pipeline. Comparisons with the same information at an earlier epoch then allows an assessment of pipeline distortion to be made.
Another concept applicable to pipeline distortion monitoring, described by Lara (U.S. Pat. No. 4,799,391), utilizes a set of angular rate measurements from a triad of three orthogonal gyros, and a set of linear acceleration measurements from a triad of orthogonal accelerometers to derive the curvature parameters of the pipeline along its entire course. Then, comparison of these curvature parameters with the same parameters at an earlier epoch provides an indication of pipeline geometric stability.